U.S. patent number 5,780,875 [Application Number 08/705,524] was granted by the patent office on 1998-07-14 for hybrid optical integration assembly using optical platform.
This patent grant is currently assigned to Hitachi Cable, Ltd., Hitachi, Ltd., Hitachi Tohbu Semiconductor, Ltd., Nippon Telegraph and Telephone Corporation. Invention is credited to Satoshi Aoki, Satoru Kikuchi, Masato Shishikura, Ryuta Takahashi, Shinji Tsuji.
United States Patent |
5,780,875 |
Tsuji , et al. |
July 14, 1998 |
Hybrid optical integration assembly using optical platform
Abstract
An optical assembly structure includes, a semiconductor element
generating a large amount of heat and a high impedance optical
element which are to be mounted, with low optical loss on the same
semiconductor substrate which has an optical waveguide formed
thereon.The element generating a large amount of heat is mounted on
a terrace of the semiconductor substrate directly or through an
insulating layer having a thickness of submicron order, while the
high impedance element is mounted in a groove which is etched into
the semiconductor substrate to such an extent that the optical axis
of the high impedance element mates with the optical axis of the
optical waveguide layer formed on a recess in the semiconductor
substrate. The optical axes can be adjusted independently for the
respective elements. Alternatively, with a potential on the
semiconductor substrate set equal to a supply voltage or a ground
potential, a crystal substrate for a forward biased heat generating
element is given the polarity opposite to that of a crystal
substrate for a reverse biased element which may imply a problem of
stray capacitance.
Inventors: |
Tsuji; Shinji (Hidaka,
JP), Takahashi; Ryuta (Hitachi, JP),
Shishikura; Masato (Hachioji, JP), Kikuchi;
Satoru (Kokubunji, JP), Aoki; Satoshi (Chigasaki,
JP) |
Assignee: |
Hitachi, Ltd. (Tokyo,
JP)
Hitachi Cable, Ltd. (Tokyo, JP)
Hitachi Tohbu Semiconductor, Ltd. (Saitama-ken,
JP)
Nippon Telegraph and Telephone Corporation (Tokyo,
JP)
|
Family
ID: |
16766671 |
Appl.
No.: |
08/705,524 |
Filed: |
August 29, 1996 |
Foreign Application Priority Data
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|
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Aug 30, 1995 [JP] |
|
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7-221433 |
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Current U.S.
Class: |
257/81; 257/84;
257/98; 257/99; 385/83; 385/92 |
Current CPC
Class: |
G02B
6/4224 (20130101); G02B 6/4246 (20130101); G02B
6/4249 (20130101); H01L 2224/45144 (20130101); H01L
2224/48091 (20130101); H01L 2924/3011 (20130101); H01L
2224/73265 (20130101); H01L 2224/49175 (20130101); H01L
2224/48091 (20130101); H01L 2924/00014 (20130101); H01L
2224/45144 (20130101); H01L 2924/00014 (20130101); H01L
2924/3011 (20130101); H01L 2924/00 (20130101) |
Current International
Class: |
G02B
6/42 (20060101); H01L 027/15 () |
Field of
Search: |
;257/80,81,84,98,99
;385/49,83,92,131 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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3731311A |
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Mar 1989 |
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DE |
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4112471A |
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Oct 1992 |
|
DE |
|
Other References
Electronics letters, "Optoelectronic Hybrid Integrated Laser Diode
Module Using Planar Lightwave Circuit Platform", by S. Mino, et
al., vol. 30, No. 22, Oct. 27, 1994. .
"Alignment-free Photodetector-Single-Mode Fiber Coupling Using a
Planarized Si Platform", 5.sup.th Optoelectronic Conference
Technical Digest, Jul. 1994, Tabuchi et al. .
"Silica-on-Terraced-Silicon Platform for Optical Hybrid
Integration" Yamada et al., 5.sup.th Optoelectronic Conference
Technical Digest, Jul. 1994. .
"A Hybrid Integrated Optical WDM Transmitter/Receiver Module for
Optical Subscriber Systems Utilizing a Planar Lightwave Circuit
Platform", Y. Yamada et al, Optical-Fiber Communication Conference
in 1995, pp. PD12-1 to PD12-5, 1995..
|
Primary Examiner: Tran; Minh-Loan
Attorney, Agent or Firm: Antonelli, Terry, Stout &
Kraus, LLP
Claims
What is claimed is:
1. An optical assembly, comprising:
a semiconductor substrate;
a semiconductor light emitting element mounted on said
semiconductor substrate; and
a semiconductor light receiving element mounted on said
semiconductor substrate,
wherein said semiconductor light emitting element is mounted on
said semiconductor substrate directly or through a thin film
layer,
said semiconductor light receiving element is mounted on said
semiconductor substrate through a dielectric layer,
said semiconductor substrate includes an optical waveguide formed
thereon, said optical waveguide having said dielectric layer,
wherein a material inserted between said semiconductor light
receiving element and said semiconductor substrate has a thickness
which is greater than the thickness of a material inserted between
said semiconductor light emitting element and said semiconductor
substrate, and
said optical waveguide has its optical axis adjusted to the optical
axis of said semiconductor light emitting element, and said optical
waveguide has its optical axis adjusted to the optical axis of said
semiconductor light receiving element.
2. An optical assembly according to claim 1, wherein said
semiconductor substrate has a terrace for mounting said
semiconductor light emitting element thereon;
said semiconductor light emitting element is mounted on said
terrace;
said semiconductor substrate has a bottom around said terrace;
said optical waveguide is formed on said bottom; and
said semiconductor light receiving element is mounted in a groove
formed by partially etching said dielectric layer.
3. An optical assembly according to claim 2, wherein said groove in
which said optical light receiving element mounted is covered by a
lid.
4. An optical assembly according to claim 2, wherein the interior
of said groove in which said semiconductor light receiving element
is mounted is filled with a resin to bury a light path.
5. An optical assembly according to claim 1, wherein said
semiconductor light receiving element has an optical waveguide
structure, and the height of the optical axis of said semiconductor
light receiving element is adjusted to the height of the optical
axis of said optical waveguide with an offset equal to or less than
3 microns.
6. An optical assembly according to claim 1, wherein said
semiconductor light receiving element is applied with a
predetermined voltage such that said semiconductor light receiving
element is reversely biased.
7. An optical assembly according to claim 1, wherein a material for
said semiconductor substrate is silicon having a crystal
orientation of (100) with an offset less than 5 degrees.
8. An optical assembly according to claim 1, wherein a dielectric
material of said optical waveguide includes silicon oxide.
9. An optical assembly according to claim 1, wherein said
semiconductor light emitting element has an optical waveguide
structure, and the height of the optical axis of said semiconductor
light emitting element is adjusted to the height of the optical
axis of said optical waveguide with an offset equal to or less than
3 microns.
10. An optical assembly according to claim 1, wherein said optical
waveguide connected to said semiconductor light emitting element is
optically coupled to said optical waveguide connected to said
semiconductor light receiving element.
11. An optical transmission module using the optical assembly
according to claim 1.
12. An optical assembly, comprising:
a semiconductor substrate;
a semiconductor light emitting element mounted on said
semiconductor substrate; and
a semiconductor light receiving element mounted on said
semiconductor substrate;
wherein said semiconductor light emitting element is mounted on
said semiconductor substrate directly or through a thin film
layer,
said semiconductor light receiving element is mounted on said
semiconductor substrate directly or through a thin film layer,
said semiconductor light emitting element is mounted to be biased
in a forward direction, and said semiconductor light receiving
element is mounted to be biased in a reverse direction,
said semiconductor light emitting element and said semiconductor
light receiving element are mounted so that signal lines for said
semiconductor light emitting element and said semiconductor light
receiving element are arranged at a side a distance from said
semiconductor substrate,
an optical waveguide having a dielectric layer is formed on said
semiconductor substrate, and
said optical waveguide has its optical axis adjusted to the optical
axis of said semiconductor light emitting element, and said optical
waveguide has its optical axis adjusted to the optical axis of said
semiconductor light receiving element.
13. An optical assembly according to claim 12, wherein said
semiconductor light emitting element and said semiconductor light
receiving element are mounted on terraces formed on said
semiconductor substrate.
14. An optical assembly according to claim 13, wherein an
insulating film is formed on each of said terraces.
15. An optical assembly according to claim 12, wherein said
semiconductor light receiving element has an optical waveguide
structure, and the height of the optical axis of said semiconductor
light receiving element is adjusted to the height of the optical
axis of said optical waveguide with an offset equal to or less than
3 microns.
16. An optical assembly according to claim 12, wherein a material
for said semiconductor substrate is silicon having a crystal
orientation of (100) with an offset less than 5 degrees.
17. An optical assembly according to claim 12, wherein a dielectric
material of said optical waveguide includes silicon oxide.
18. An optical assembly according to claim 12, wherein said
semiconductor light emitting element has an optical waveguide
structure, and the height of the optical axis of said semiconductor
light emitting element is adjusted to the height of the optical
axis of said optical waveguide with an offset equal to or less than
3 microns.
19. An optical assembly according to claim 12, wherein said optical
waveguide connected to said semiconductor light emitting element is
optically coupled to said optical waveguide connected to said
semiconductor light receiving element.
20. An optical assembly according to claim 12, wherein said
semiconductor light receiving element is mounted on a groove formed
on said semiconductor substrate, said groove being covered by a
lid.
21. An optical assembly according to claim 12, wherein said
semiconductor light receiving element is mounted on a groove formed
on said semiconductor substrate, and an interior of said groove and
an optical path are filled with a resin.
22. An optical transmission module using the optical assembly
according to claim 12.
23. An optical assembly comprising:
a semiconductor substrate;
a pre-amplifier element mounted on said semiconductor substrate;
and
a semiconductor light receiving element mounted on said
semiconductor substrate,
wherein said pre-amplifier element is mounted on said semiconductor
substrate directly or through a thin film layer,
said semiconductor light receiving element is mounted on said
semiconductor substrate through a dielectric layer,
wherein a material inserted between said semiconductor light
receiving element and said semiconductor substrate has a thickness
which is greater than the thickness of a material inserted between
said pre-amplifier element and said semiconductor substrate,
and
said semiconductor substrate includes an optical waveguide formed
thereon, said optical waveguide including said dielectric
layer.
24. An optical assembly, comprising:
a semiconductor substrate;
an element generating a large amount of heat mounted on said
semiconductor substrate; and
a high impedance optical element mounted on said semiconductor
substrate,
wherein said element generating a large amount of heat is mounted
on said semiconductor substrate directly or through a thin film
layer,
said high impedance optical element is mounted on said
semiconductor substrate through a dielectric layer,
said semiconductor substrate includes an optical waveguide formed
thereon, said optical waveguide including said dielectric
layer,
wherein a material inserted between said high impedance optical
element and said semiconductor substrate has a thickness which is
greater than the thickness of a material inserted between said
element generating a large amount of heat and said semiconductor
substrate, and
said optical waveguide has its optical axis adjusted to the optical
axis of said element generating a large amount of heat, and said
optical waveguide has its optical axis adjusted to the optical axis
of said high impedance optical element.
25. An optical assembly, comprising:
a semiconductor substrate;
an element generating a large amount of heat mounted on said
semiconductor substrate; and
a high impedance optical element mounted on said semiconductor
substrate,
wherein said element generating a large amount of heat is mounted
on said semiconductor substrate directly or through a thin film
layer,
said high impedance optical element is mounted on said
semiconductor substrate directly or through a thin film layer,
said element generating a large amount of heat is forward biased,
and said high impedance optical element is reverse biased,
said element generating a large amount of heat and said high
impedance optical element are mounted so that signal lines for said
element generating a large amount of heat and said high impedance
optical element are arranged at a side a distance from said
semiconductor substrate,
an optical waveguide having a dielectric layer is formed on said
semiconductor substrate, and
said optical waveguide has its optical axis adjusted to the optical
axis of said element generating a large amount of heat, and said
optical waveguide has its optical axis adjusted to the optical axis
of said high impedance optical element.
26. An optical assembly, comprising:
a semiconductor substrate;
an element generating a large amount of heat mounted on said
semiconductor substrate; and
a high impedance optical element mounted on said semiconductor
substrate,
wherein said element generating a large amount of heat is mounted
on said semiconductor substrate directly or through a thin film
layer,
said high impedance optical element is mounted on said
semiconductor substrate through a dielectric layer,
wherein a material inserted between said high impedance optical
element and said semiconductor substrate has a thickness which is
greater than the thickness of a material inserted between said
element generating a large amount of heat and said semiconductor
substrate, and
said semiconductor substrate includes an optical waveguide formed
thereon, said optical waveguide including said dielectric layer.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a low cost optical module
structure which is applied to optical access networks, optical
exchange systems, optical interconnects, and so on.
There are several reports on hybrid integration of laser diodes
and/or photodiodes on optical platforms. For example, H. Tabuchi,
et al. reported "alignment-free photodetector single-mode fiber
coupling using a planarized Si platform" (paper 15B1-3) at the
fifth optoelectronics conference held at Chiba, Japan in 1994. At
the same conference, Y. Yamada reported "silica-on-terraced-silicon
platform for optical hybrid integration" (paper 15B1-3).
Further, there is a paper by Yamada et al entitled "A Hybird
Integrated Optical WDM Transmitter/Receiver Module for Optical
Subscriber Systems Utilizing a Planar Lightwave Circuit Platform",
pages PD12-1 to PD12-5, reported at the Optical-fiber Communication
Conference in 1995 .
FIG. 1A is a top plan view illustrating the structure of a prior
art optical assembly, and FIG. 1B is a cross-sectional view taken
along a line I-I' in FIG. 1A. The illustrated structure includes a
semiconductor laser 2 and waveguide-type photodiodes 3, each formed
on an n-type substrate, which are mounted on a silicon substrate 10
having a silica waveguide 1 formed on the upper surface thereof.
The optical waveguide 1 includes an under cladding layer 12 made of
silica embedded in a recess ( portion) of the silicon substrate 10,
an adjusting layer 13 made of silica formed on the under cladding
layer 12, an optical waveguide core 14 having a refractive index
larger than that of the under cladding layer 12, and an upper
cladding layer 15 made of silica. The semiconductor laser 2 and the
waveguide-type photodiodes 3 are fixed opposite to a common port
for optical input and output of the optical waveguide 1 on
electrodes 720, 730 on a place for mounting 5 formed by selectively
etching glass on a terrace ( portion) 11 of the silicon substrate
10.
Since the optical waveguide 1 and the optical elements 2, 3 are
required to be optically connected to each other with low loss,
techniques such as a fine positioning control using index marks,
self-alignment utilizing surface tension of solder bump, and so on
have been proposed as low cost mounting methods in order to adjust
the optical axes between the optical waveguide core 14 and the
mounted elements in the plane of Si bench. In addition, several
designs for reducing the influence of misalignment have been
included in the optical assembly, for example, a mounted optical
element is selected such that its spot size is close to that of a
silica waveguide.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical
assembly structure which is capable of mounting an element
generating a large amount of heat and a high impedance optical
element on the same optical waveguide substrate with the optical
axes thereof mated with the optical axis of an optical waveguide at
a low cost.
To achieve the above object, an element generating a large amount
of heat is fixed on a terrace of a semiconductor substrate directly
or through an insulating layer having a thickness of submicron
order, so that the element generating a large amount of heat can be
mounted with a low heat resistance. In the case of laser diodes,
the optical axes of the laser and the silica optical waveguide
should be adjusted for the best optical coupling. This adjustment
can be attained physically by setting the waveguide axis height
measured from the silicon terrace surface to the sum of the active
layer depth of the laser diode and the thicknesses of an electrode
and solder. When the high impedance optical element is mounted in a
groove which is etched into the semiconductor substrate to such an
extent that the optical axis of the high impedance optical element
mates with the optical axis of the optical waveguide formed on a
recess of the semiconductor substrate, the high impedance optical
element can be mounted substantially free from any electrical
influence of the conductive silicon substrate.
As another approach, it is also possible to equivalently reduce a
stray capacitance of the high impedance optical element, including
a stray capacitance caused by a connection with electric circuits.
Specifically, the semiconductor substrate is set at the same
potential as a supply voltage or a grounding potential, and signal
lines of two or more mounted elements are kept away from the
semiconductor substrate. This structure can be realized by mounting
a forward biased heat generating element and a reverse biased high
impedance optical element, causing a problem of a stray
capacitance, with their polarities opposite to each other.
With the foregoing approaches, the element generating a large
amount of heat and the high impedance element can be mounted on the
same optical waveguide substrate with their optical axes mated with
the optical axis of the waveguide at a low cost.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A, 1B are a top plan view and a cross-sectional view
respectively illustrating the structure of a prior art optical
assembly;
FIGS. 2A, 2B are a top plan view and a cross-sectional view
respectively illustrating a structure associated with a first and a
fourth embodiments of the present invention;
FIG. 3 is a graph showing an optical output characteristic
associated with the first embodiment of the present invention;
FIGS. 4A, 4B are a top plan view and a cross-sectional view
respectively illustrating a structure associated with a second
embodiment of the present invention;
FIG. 5 is a top plan view illustrating an example in which the
present invention is applied to a second transmission board;
FIGS. 6A, 6B are a top plan view and a cross-sectional view
respectively illustrating a structure associated with a third
embodiment of the present invention; and
FIGS. 7A, 7B are a top plan view and a cross-sectional view
respectively illustrating a structure associated with a fifth
embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiment 1
FIG. 2A is a top plan view illustrating the structure of a first
embodiment of the present invention, and FIG. 2B is a
cross-sectional view taken along a line II-II' in FIG. 2A. On a
silicon substrate 10 having a terrace 11 on the upper surface
thereof, an under cladding layer 12 made of silica is formed in a
thickness sufficiently larger than the height of the terrace 11
(20-30 m) using an electron beam deposition. Here, a silicon having
a crystal orientation of (100) with an offset less than 5 degrees
is used such that the terrace 11 has a symmetric shape. Thereafter,
the under cladding layer 12 is polished until the upper surface of
the terrace of the silicon substrate 10 is completely exposed to
planarize the terrace 11 of the silicon substrate 10 and the under
cladding layer 12. On the planarized upper surfaces of the terrace
11 and the under cladding layer 12, silica containing titanium,
germanium, and so on is deposited by electron beam deposition and
patterned to form an optical waveguide core 14. Next, silica free
from additives is deposited by electron beam deposition on the
optical waveguide core 14 to form an upper cladding layer 15. Then,
the upper cladding layer 15 and the optical waveguide core 14 are
partially dry-etched using fluorine-based gas until the upper
surface of the terrace 11 of the silicon substrate 10 is exposed.
Since the etching is stopped when the surface of the silicon
substrate 10 is exposed, excessive etching would not cause any
problem. Rather, the dry etching may be stopped at the time the
depth of a recess serving as a second place for mounting 53 reaches
a point at which the optical axis 31 of a waveguide type photodiode
3, to be mounted in the recess, mates with the optical axis of the
optical waveguide core 14. Since the etching rate can be controlled
at approximately 0.1 -1 .mu.m per minute, the depth of the recess
can be controlled with good reproducibility by controlling etching
time. Since the length from the upper surface of the second place
for mounting 53 to the silicon substrate 10 is 10 micron or more,
an increase in stray capacitance in the waveguide type photodiode 3
through the silicon substrate 10 is 0.02 pF or less.
Next, since the optical axis of the optical waveguide core 14 in a
first place for mounting 52 does not mate with the optical axis 21
of a semiconductor laser 2, the silicon substrate 10 is selectively
etched to adjust only the height of the first place for mounting 52
without affecting the second place for mounting 53. In this way,
the optical axes of the two places for mounting 52, 53 can be
adjusted independently of each other. Next, electrode layers made
of titanium, platinum, and gold are formed on the respective places
for mounting 52, 53 by electron beam deposition and patterned in
compliance with the shapes of electrodes to form electrodes 720,
721, 722, 730, 731, 732, thus fabricating an optical waveguide
substrate 100. In the first embodiment, the optical waveguide 1 has
a Y-shape in the top plan view, where one terminal is utilized as a
common port for optical input and output and two branched terminals
are utilized as a port for connecting to the semiconductor laser 2
on the first place for mounting 52 and a port for connecting to the
waveguide type photodiode 3 on the second place for mounting 53,
respectively. Index patterns for mounting the elements are made
simultaneously with the formation of the electrode pattern. In
addition, a crystal substrate 20 of the semiconductor laser 2 and a
crystal substrate 30 of the waveguide type photodiode 3, used for
the mounting, are both made of n-type InP crystal.
Next, the semiconductor laser 2 is mounted on the electrode 720
through an AuSn thin-film solder 82 (having a thickness ranging
from 1 to 6 .mu.m) by detecting index patterns using transmission
infrared light illuminated from the backside of the silicon
platform. An index pattern stamped on the semiconductor laser 2 and
an index pattern stamped on the upper surface of the silicon
substrate forming the first place for mounting 52 are
simultaneously transmitted by infrared light, observed by an
infrared TV camera to detect relative misalignment between the
patterns, and aligned. After the alignment, the solder 82 is melted
by heating to thereby fix the semiconductor laser 2 thereon. Then,
64 a similar method, the waveguide type photodiode 3 is fixed on
the second place for mounting 53 using solder 83. The electrode 720
is connected to the electrode 721 by an Au wire 9 while the
semiconductor laser 2 is connected to the electrode 722 by an Au
wire 9 to establish electric connections of the semiconductor laser
2. Similarly, the electrode 730 is connected to the electrode 731
by an Au wire 9 while the waveguide type photodiode 3 is connected
to the electrode 732 by n Au wire 9 to establish electric
connections of the waveguide type photodiode 3.
After the trial manufacturing of the optical assembly, a fiber is
connected to the common port for optical input and output of the
optical assembly to evaluate the optical output characteristic and
sensitivity characteristic of the optical assembly. FIG. 3 shows
the optical output characteristics of three optical assemblies.
When the semiconductor laser 2 is driven, a fiber optical output of
1 mW is generated with an operating current at 50 mA. It can be
confirmed that a positioning error after mounting the semiconductor
laser 2 can be controlled within 1 .mu.m by comparing the optical
output characteristic of the semiconductor laser 2 before being
mounted. Also, for a light receiving sensitivity, a sufficient
sensitivity characteristic of 0.31 A/W is obtained. A positioning
error of the waveguide type photodiode 3 is also within 1 .mu.m. In
addition, from the result that the thermal resistance of the
semiconductor laser is below 40.degree. C./W, it is confirmed that
a sufficient heat releasing characteristic can be ensured. The
capacitance of the photodiode is not more than 1 pF, including a
capacitance due to the mounting thereof.
Embodiment 2
FIG. 4A is a top plan view illustrating a second embodiment of the
present invention, and FIG. 4B is a cross-sectional view taken
along a line IV-IV' in FIG. 4A. An under cladding layer 12 made of
silica is formed on a silicon substrate 10 having terraces 11 on
the upper surface thereof in a thickness sufficiently larger than
the height of the terrace 11 (20-30 .mu.m) using electron beam
deposition. Here, a silicon having a crystal orientation of (100)
with an offset less than 5 degrees is used such that each of the
terraces 11 has a symmetric shape. Thereafter, the under cladding
layer 12 is polished until the upper surface of the silicon
substrate 10 is completely exposed to planarize the terraces 11 of
the silicon substrate 10 and the under cladding layer 12. An
adjusting layer 13 made of silica is deposited on the planarized
upper surfaces of the terraces 11 and the under cladding layer 12,
and then silica containing titanium, germanium, and so on is
deposited by electron beam deposition and patterned to form an
optical waveguide core 14. Next, silica free from additives is
deposited by electron beam deposition on the optical waveguide core
14 to form an upper cladding layer 15. Thereafter, the upper
cladding layer 15 and the optical waveguide core 14 are partially
dry-etched using fluorine-based gas until the upper surfaces of the
terraces 11 of the silicon substrate 10 are exposed. Since the
etching is stopped when the surface of the silicon substrate 10 is
exposed, excessive etching would not cause any problem. The
adjusting layer 13 is formed to mate the optical axis of the
optical waveguide core 14 with the respective optical axes 21, 31
of a semiconductor laser 2 and a waveguide type photodiode 3.
Next, after silicon oxide films 16 having a thickness equal to or
less than 0.5 micron are formed on places for mounting 52, 53,
respectively, electrode layers made of titanium, platinum, and gold
are formed by electron beam deposition and patterned in compliance
with electrode shapes to fabricate an optical waveguide substrate.
Index patterns for mounting elements are stamped simultaneously
with the formation of the electrode pattern. Also in the second
embodiment, the optical waveguide 1 has a Y-shape in the top plan
view, where one terminal is utilized as a common port for optical
input and output and two branched terminals are utilized as a port
for connecting to the semiconductor laser 2 on the first place for
mounting 52 and a port for connecting the waveguide type photodiode
3 on the second place for mounting 53, respectively.
Next, the semiconductor laser 2 is mounted on the electrode 720
through an AuSn thin-film solder 82 (having a thickness ranging
from 1 to 6 .mu.m) by detecting index marks using transmission
infrared light illuminated from the backside of the silicon
platform. An index pattern stamped on the semiconductor laser 2 and
an index pattern stamped on the upper surface of the silicon
substrate forming the first place for mounting 52 are
simultaneously transmitted by infrared light, observed by an
infrared TV camera to detect relative misalignment between the
patterns, and aligned. After the alignment, the solder 82 is melted
by heating to fix the semiconductor laser 2 thereon. Then, by a
similar method, the waveguide type photodiode 3 is fixed on the
second place for mounting 53 using solder 83. After mounting the
elements, a lid 61 made of glass is placed overlying grooves, and
fixed by a resin 62. The use of the lid protects the elements from
damage during a fiber connection process and enables stable trial
manufacturing of the optical assemblies.
In the second embodiment, unlike the first embodiment, an n-type
InP crystal is used for the crystal substrate 20 of semiconductor
laser 2, while p-type InP crystal is used for crystal substrate 30
of the waveguide type photodiode 3. In this way, the silicon
substrate 10 is set at the same potential as a power supply, and
signal lines for the waveguide type photodiode 3 can be placed on
the crystal substrate side, whereby an increase in stray
capacitance can be reduced to 0.3 pF or less.
As seen in FIG. 5, after, the trial manufacturing of the optical
assembly 100, a fiber 104 is connected to the common port for
optical input and output of the optical assembly 100 using a glass
block 103 to evaluate the optical output characteristic and
sensitivity characteristic. As a result, similar characteristics to
those of the first embodiment were obtained. Then, the optical
assembly 100 is mounted on a printed circuit board 101 together
with a laser driving IC 105 and a pre-amplifier IC 106. FIG. 5
illustrates how the optical assembly is mounted on the printed
circuit board 101. Two sets of such trial boards are prepared and
placed opposite to each other to evaluate the transmission
characteristic. With a transmission rate at 30 Mb/s and a
transmission length extending 5 km, it is confirmed that the
transmission can be performed with a bit error rate (BER) equal to
or less than 10 to the minus ninth power(10-.sub.9).
Embodiment 3
FIG. 6A is a top plan view illustrating a third embodiment of the
present invention, and FIG. 6B is a cross-sectional view taken
along a line VI-VI' in FIG. 6A. Instead of using the lid 61 for
covering the mounted elements as in the second embodiment, the
third embodiment fills an epoxy resin 63 in grooves for protecting
elements 2, 3. The filled epoxy resin 63 substantially completely
prevents the characteristics of the elements from deteriorating.
Eleven sets of modules each having the optical assembly of the
third embodiment mounted thereon are left in a test chamber at a
temperature of 85.degree. C. and a relative humidity of 90% RH to
evaluate deteriorations in the optical output characteristic and
optical sensitivity characteristic. After a test over 2000 hours,
none of the 11 modules exhibit deteriorated characteristics. On the
other hand, when the same test is conducted on an optical module
without the filled resin for protection, the results showed that
after 200-500 hours, a dark current in the photodiode 3 abruptly
increases, thus confirming the validity of the filled resin of the
third embodiment.
Embodiment 4
In a fourth embodiment of the present invention, the material for
the optical waveguide is changed from silica used in the third
embodiment to polyimide. Fluorinated polyimide is used for cladding
layers 12, 13, 15, and an optical waveguide is formed such that a
refractive index difference with a core is limited to 1%. Since an
organic material is used for the optical waveguide 1, the film
formation can be completed in a shorter time by spin coating, thus
facilitating a further reduction in cost. An epoxy resin is used to
fill grooves, as is the case of the third embodiment.
Embodiment 5
FIG. 7A is a top plan view illustrating a fifth embodiment of the
present invention, and FIG. 7B is a cross-sectional view taken
along a line VII-VII' in FIG. 7A. In the fifth embodiment, a
waveguide type photodiode array 3 is optically connected to a
waveguide array 110. A trial manufacturing process of an optical
waveguide substrate used herein for mounting elements is similar to
that of the first embodiment. In the fifth embodiment, a
pre-amplifier IC array 4 having ten channels is mounted on a place
for mounting 54 formed on a terrace 11 of a silicon substrate 10,
and a waveguide type photodiode array 3 having ten channels is
mounted on a place for mounting 52 formed by etching into a glass
surface of a recess in the silicon substrate 10. The pre-amplifier
IC array 4 is made on a trial basis using an Si bipolar process
since the pre-amplifier IC array may operate at high speeds up to
approximately 1 Gb/s. The waveguide type photodiode array 3 is
connected to an optical waveguide core 14 having an array of ten
channels.
Ten channels of fibers are connected to the optical assembly of the
fifth embodiment to measure a light receiving sensitivity. As a
result, the optical assembly exhibits a light receiving
characteristic with small variations between channels at
0.85.+-.0.02 A/ W. Also, the optical assembly is evaluated in terms
of a receiver sensitivity at a rate of 200 bits/second for each
channel over a transmission length extending 100 m. On each
channel, error free operation continues for fifty hours or more.
Since the application of the waveguide type photodiode facilitates
the mating of an electric wire face with a light incident face, a
simple module can be realized.
In the present invention, each of the heights of the optical axes
of the semiconductor light receiving element 3 and semiconductor
light emitting element 2 is adjusted to the height of the optical
axis of the optical waveguide 1 with an offset equal to or less
than 3 microns.
According to the present invention, an element generating a large
amount of heat and a high impedance optical element can be mounted
on the same optical waveguide substrate with the optical axes
thereof mated with the optical axis of an optical waveguide. In
this way, a hybrid optical circuit having a variety of optical
elements and electric elements mounted on a waveguide can be
mounted at a low cost. Thus, large capacity communications
supporting multimedia environments can be accomplished by laying
optical fibers to respective houses. In addition, since the optical
assembly of the present invention allows for the introduction of
optical fibers for wiring between apparatuses, larger scale
parallel processing can be readily realized.
Many different embodiments of the present invention may be
constructed without departing from the spirit and scope of the
invention. It should be understood that the present invention is
not limited to the specific embodiments described in this
specification. To the contrary, the present invention is intended
to cover various modifications and equivalent arrangements included
within the spirit and scope of the claims.
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